Rechargeable electrochemical cell pack having overcurrent protection

ABSTRACT

A rechargeable electrochemical cell pack is provided having a casing holding a plurality of electrically interconnected cells and having circuit breaking means to guard against the possibility of excessive supply and consumption of electrical power and the risk of damage or injury incident thereto. The circuit breaker comprises a small current fuse contained internally of the cell pack so as to give an external visible indication of its state and is removably mounted on an access door for replacement purposes.

FIELD OF THE INVENTION

The present invention relates to rechargeable electrochemical cellpacks;

more specifically, it relates to rechargeable electrochemical cell packshaving self-contained circuit breaker means for preventing excessivecharge and discharge rates for the cell circuit.

BACKGROUND OF THE INVENTION

Rechargeable electrochemical cells, such as nickelcadmium cells, havebeen assimilated to a wide range of end uses. Such cells are nowcommonplace not only in industrial environments, but on the massconsumer market. As the scope of use has expanded, so has the likelihoodof misuse and misapplication of the cells by the user, either during thedischarge or charge cycle.

Among the problems caused by excessive current flow through the cellsare those of cell degradation or, in the case of malfunction of a ventedcell, dangerous overpressure and overtemperature conditions in the cell.These conditions present the risk of damage being caused to the cell orto equipment in electrical connection with the cell. Indeed, theseconditions may even present the possibility of injury being caused to auser. Such problems are amplified by the use of several interconnectedelectrochemical cells which produce higher voltages and higher currentdrain capacity. Moreover, the use of several individual cells in asingle battery unit (cell pack) increases the probability ofmalfunction.

Encasing the cell pack of electrochemical cells in a material capable ofwithstanding appreciable amounts of heat and pressure can provide asolution to these difficulties. However, such a casing would detractfrom the overall convenience and efficiency of the cell pack. Fusing thecell pack by conventional techniques such as placing a fuse in thevicinity of the power source or permanently soldering a fuse into placealso presents difficulties because the portable nature of the cell packsrequires a compact source structure which can be readily maintained.

Although most cells are equipped with small built-in pressure releasevalves to vent internal gases in the event of an overpressure condition,the possibility still remains that an overpressurized cell may causedamage to the cell pack or to the equipment the cell pack powers or isbeing charged with. Moreover, cell performance can be seriouslydowngraded in the event of such overpressure conditions. For example,during a venting period early in the cell life, some of the electrolytein minature droplet form can be entrained in the vented gas stream.Should repeated venting occur due to pressure increases at high currentloadings, a significant loss of electrolyte and gases is likely.

Excessive cell temperature conditions are aggravated whenever aplurality of cells are grouped together, since each cell is in effect aheat generating source. When placed in a common container or package,the rate of dissipation of internally generated heat to the environmentis reduced, and the cells operate at higher equilibrium temperatures.Such temperatures are directly related to the amount of current flowingthrough the cell during charging and discharging. As current increases,heat generated through the cell's internal resistance R_(c) increasesaccording to the formula I² R_(c). Not only do excessive temperaturepromote excessive cell pressures and the risk of damage or injuryincident thereto, prolonged exposure of the cell to high temperatureshastens decomposition of the separator and seal materials.

The foregoing considerations highlight the need for an effective meansof overcurrent protection which will preclude excessive current flowthrough a multi-cell battery pack.

Among the objects of the invention, therefore, is to provide arechargeable electrochemical cell pack having circuit breaker means forprotecting the cell pack and its surrounding equipment againstovercurrent conditions.

Another object of the invention is to provide rechargeableelectrochemical cell packs having circuit breaker means which can bemanufactured cheaply and in a compact configuration which does notincrease the overall dimensions of the unit.

These and other objects will become apparent from the following summaryand description of the invention taken in conjunction with theaccompanying drawings.

SUMMARY OF THE INVENTION

Broadly contemplated, the present invention provides a rechargeableelectrochemical cell pack having self-contained overcurrent protection.The cell pack comprises:

(a) a casing having exterior connection terminals; and

(b) a plurality of rechargeable electrochemical cells mounted in thecasing and interconnected with the exterior terminals in a serieselectrical circuit which includes circuit breaker means.

The circuit breaking means is positioned within the casing, preferablyin an interior space between adjacent cells, and is disposed so as toprovide a visible indication at the casing exterior. In the preferredembodiment, the circuit breaking means comprises a fusible link typedevice which is carried at the underside of a removable cover so as tomate with electrical contacts at the casing interior when the cover isin place.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a preferred cell pack according to theinvention, partially broken away at the top to show the position ofindividual cells at the interior;

FIG. 2 is a plan similar to FIG. 1, with the top removed;

FIG. 3 is a perspective view of two cells of the cell pack, showing onearrangement for connecting a circuit breaking fuse in the batterycircuit;

FIG. 4 is a sectional side view taken generally along line 4--4 of FIG.2; and

FIG. 5 is a perspective view looking upward on a removeable doorassociated with the FIG. 2 cell pack.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like numerals are used to indicatelike parts throughout the various figures, FIG. 1 illustrates thepreferred form of rechargeable cell pack 10 according to the invention.It is understood that the term "cell pack" denotes a unitary packageincluding two or more individual electrochemical cells which areelectrically interconnected, usually in series.

Cell pack 10 is a rechargeable system capable of providing electricalpower for a wide variety of purposes. Depending upon the number ofinternal individual cells and their manner of interconnection, any of awide variety of voltages and current ratings can be provided.

For purpose of illustration, five electrochemical storage cells areshown in cell pack 10. Nickel-cadmium cells have a nominal operatingvoltage of 1.25 V.D.C. Thus, the overall cell pack terminal voltage inthis case will be about 6 V.D.C. A typical cell is the AA size cellwhich, in physical dimension, measures about 0.59 inches in diameter andabout 1.95 inches in longitudinal length, and has a rated capacity of500 mAh. It will be understood, of course, that the invention isapplicable to cells of all sizes and ratings. Cylindrical sealed cellsrange from about 0.54 inch to about 1.27 inches in diameter and fromabout 0.6 inch to about 3.4 inches in length. Such cells are rated fromabout 0.1 Ah to about 5.6 Ah. Suitable nickel-cadmium cells and theircharge and discharge characteristics are described in the Nickel-CadmiumBattery Application Engineering Handbook, published by General ElectricCompany, Battery Business Department, Gainsville, Fla.

Cell pack 10 is rectangular in shape and generally includes casing 16which houses the five electrochemical cells 18 and a cover assembly 19.This cover assembly includes a removable inspection door 14 having aviewing port, or window 15, through which can be seen a visibleindication of the state of a circuit breaker element 12 (e.g., a fuse).Casing 16 and cover 19 can be fabricated from a wide variety ofmaterials suitable for housing electrical equipment, preferablypolystyrene. Positioned on end wall 20 of the casing are terminalconnectors 22 and 24. These terminals can be of the pressure contacttype or they can be conventional snap type connectors used for makingelectrical contact with the equipment to be powered as well as therecharging equipment (neither shown). Terminals 22 and 24 can have awide variety of shapes and sizes. Also positioned on side wall 20 is aprojecting rib 25 used to align a terminal strip or rechargingequipment.

As best seen in FIG. 2, cells 18 are arranged side by side in thehousing 16 so that the adjacent cells abut or nearly abut; the packthickness is thus principally determined by the diameter of the cells.The cells are provided with insulating sleeves 17 (FIG. 3) to precludeshorting of adjacent cell casings when the cells are series connected,since the casings also constitute the negative battery terminals. Inorder to prevent cells 18 from shifting, cell pack casing 16 is providedwith spacers 26 which extend inwardly into the interior space of thecell pack. If desired, these spacers can be made resilient to absorbshock and reduce cell movement, or they can be contoured to conform tothe curved outline of the abutting cell.

As indicated above, the cells 18 are connected in series. Although anumber of cell positioning arrangements can be employed, the preferredmethod is to arrange the cells so that the terminal polarity alternatesfrom one cell to the next, with the exception of two middle cells whoseterminal polarities match. By arranging the cells in this manner, allbut one of the series connections between the cells are affected bystrap connectors 28 positioned along the longitudinal side walls 30 and32 of casing 16. Thus, as will be seen in FIG. 1, three connectors 28are employed to make all but one of the connections between the cells.Connectors 28 are conventional and can be simple metal clips supportedby casing 16, or they can be sheet metal strips welded directly to thecell ends, as shown in FIGS. 3 and 4. To complete the circuit, terminals22 and 24 are connected to opposite poles of the first and last cells ofthe series circuit by wire leads 34 and 36 (FIG. 2).

As indicated earlier, the series cell circuit within the cell pack isprovided with circuit breaking means to preclude its supply orconsumption of excessive power. This circuit breaker means can beadapted to operate over a wide range of power levels to prevent the cellpack and the surrounding equipment from being damaged. Generally, thecircuit breaker means will be adapted so that the cell circuit will bebroken at power levels which are unduly high and which prevent asubstantial risk of damage being caused to the cell pack and toequipment in electrical connection with the cell pack. In this way,although current levels may be reached which may cause some damage toindividual cells, power levels will not be reached which would presentthe risk of damage to equipment or injury to the user.

The circuit breaking means is so located that it does not increase theoverall dimensions of the cell pack, is not costly to manufacture, isreadily visible for inspection by the user, does not appreciablyincrease the internal impedance of the battery circuit and yet isreliable and fatigue resistant.

Considering now FIGS. 2-4, the circuit breaking fuse 12 is seen topartially occupy the void 35 formed between adjacent cells 18A, 18B, andto partially occupy a recess 33 formed in the removable cover 14.Turning momentarily to FIG. 5, the manner of mounting fuse 12 will beapparent. The removable cover 14, or door, is provided with spring clipfuse holders 37 which extend downwardly from the door's underside. Fuse12 is positioned on door 14 so that, when the door is fitted into casing16, fuse contact ends 12a and 12b register and make electrical contactwith mating contacts inside the casing. The clips 37 supporting the fusecan constitute projections molded integrally with the door 14 instead ofthe separate metal clips illustrated.

Returning to FIGS. 2-4, the fuse contacts inside the casing are formedfrom specially formed conductive metal strips 38, 44. Conductive strip38 is directly affixed to the casing of cell 18A by welds 39 at thenegative cell terminal whereas a similarly formed strip 44 is welded tothe positive terminal of cell 18B. For reasons discussed hereafter,conductive strips 38 and 44 are fabricated from a resilient spring-likematerial capable of retaining its shape even under long periods ofstress.

As best seen in FIGS. 2, 3 and 4, conductive strip 38 extends upwardlyfrom the cell and then longitudinally along the side of cell 18A for aportion of the cell length; it then turns perpendicularly toward cell18B and downwardly into intercell space 35 formed between cells 18A and18B. Conductive strip 38 terminates in a V-shaped end portion 42 which,as will be seen in FIGS. 3 and 4, provides a fuse receiving groovefacing upwardly toward the door 14. (In FIG. 3 the fuse is shown insolid lines to illustrate its location when the door is in place.)Conductive strip 44 is identical to conductive strip 38 and, thus, acommon manufacturing procedure can be used to form each. It, too, ispositioned so that its V-shaped portion 46 occupies the intercell spacewith its open end facing the door 14. V-shaped end portions 42 and 46are spaced so as to register with contact ends 12a and 12b of the fuse.The V-shape of resilient end portions 42 and 46 enables them to conformto the fuse contacts and to adjust for distance as the door 14 is putinto place.

As is best apparent in FIG. 4, the cells in the casing 16 may actuallycooperate with the conductive fuse contact strips 38, 44 so as to impartmechanical integrity to the fuse-to-battery connection. When the fuse isin place, it bears down on V-shaped portions 42, 46 which, in turn maybe supported by the casings of cells 18A, 18B. Thereupon resilientpressure is applied to the contact points by flexing of the arms of theV-shaped ends 42, 46. Though the V-shaped ends 42, 46 may contact thecells, they remain electrically isolated owing to the insulating sleeves17 which surround the cell casings. An additional feature of using thestrips 38, 44 is the fact that they can be welded directly to, andsupported by, the cells 18, with the cells themselves holding the stripsin position. This means that no additonal manufacturing steps arerequired for assembly of the cell pack, since the fuse contacting strips38, 44 simply take the place of an interconnecting strap 28 duringwelding operations.

It will be observed that stick type fuse 12 is accommodated partly bythe intercell space 35 and partly by the recess 33 in door 14 so as toavoid any substantial increase in the dimensions of the casing by virtueof the circuit breaker feature. Moreover, ready access to fuse 12 isprovided by mounting the fuse on the removable door so as to eliminatethe need to insert fingers or tools into the casing for the purpose offuse removal.

Directing attention to FIGS. 4 and 5, door 14 is rectangular in shapeand adapted to be snap-fitted into an opening in the top 19 of casing16. A downwardly offset lip 52 running along one edge of door 14 isdimensioned to be received in a longitudinal recess 54 in casing 16. Theopposite edge of the door is formed with a downturned portion 56terminating in a latching hook 57 which engages the underside of thefacing edge portion 59 of the casing. Because of the generally flexiblenature of the casing material, the beveled portion of hook 57 yieldsupon engaging casing edge portion 59, and then snaps resiliently to theposition shown in FIG. 4 when fully in place to hold the door 14securely in place while exerting sufficient downward pressure on thefuse 12 to form good electrical contact with the strip connectors 38, 44so as to complete the battery circuit.

In summary, the invention provides a compact cell pack arrangementwherein the battery circuit is protected against overcurrent conditions.This feature is provided in a low cost manner adapted to mass productionand is combined with the cell pack such that its size and convenienceare not disadvantageously affected. To the contrary, the cell packincludes means whereby a circuit breaking fuse is accommodated at theinterior of the cell pack such that its condition is visible at the packexterior. The use of resilient fuse contacts permanently affixed to thebattery pack and cooperating with the cells facilitates easy removal ofthe fuse and assists in the integrity of the connection.

While I have shown in the drawings and have described in detail onepreferred embodiment of the invention, it is to be understood that thisdescription is an exemplification. Certain variations and adaptions canbe made to the cell pack described above without departing from thespirit of the invention which is to be limited only by the scope of theappended claims.

What I claim is:
 1. A rechargeable electrochemical cell packcomprising:a casing having positive and negative terminals at theexterior thereof for connection to an external circuit; a plurality ofrechargeable electrochemical cells housed in said casing, at least twoof said cells defining a space between said at least two of said cells;means at the interior of the casing for interconnecting said cells in aseries circuit between said casing terminals, said series circuitincluding a pair of electrically isolated contacts, each of saidelectrically isolated contacts having a terminal receiving portion, saidterminal receiving portion being positioned in said space between saidat least two of said cells; circuit breaking means positioned withinsaid casing in generally parallel relation to said at least two of saidcells and at least partially occupying said space between said at leasttwo of said cells, said circuit breaking means being positioned betweensaid terminal receiving portions of said pair of electrically isolatedcontacts for completing said series circuit and for interrupting saidseries circuit when the current flow therethrough exceeds apredetermined level; and a removable door in said casing, said circuitbreaking means being in registering relation with said removable door sothat said circuit breaking means is firmly held in position in saidcasing when said removable door is in place.
 2. The cell pack of claim1, wherein:the casing includes a viewing port registered with saidcircuit breaking means so as to present a visible indication outside thecasing of the conductive state of said circuit breaking means.
 3. Thecell pack of claim 1,wherein said circuit breaking means is carried bysaid removable door.
 4. The cell pack of claim 3 wherein:saidelectrochemical cells are cylindrical and are mounted in said casingside by side.
 5. The cell pack of claim 4, wherein:said circuit breakingmeans comprises a stick type fuse, said fuse being positioned ingenerally parallel relation to said cells and at least partiallyoccupying the space formed between two adjacent cells.
 6. The cell packof claim 3, further comprising:means at the interior surface of saiddoor for releasably holding said circuit breaking means, said circuitbreaking means comprising a fuse.
 7. The cell pack of claim 3,wherein:said door includes an offset edge portion for engaging anundersurface at one location of the casing, and a downwardly projectinghook portion physically displaced from said offset edge portion foryieldably and releasably engaging said casing undersurface at adifferent location.
 8. The cell pack of claim 1, wherein:said isolatedcontacts comprise conductive strips affixed to opposite terminals of twoof said cells.
 9. The cell pack of claim 7, wherein:the circuit breakingmeans comprises a stick type fuse, and the terminal receiving portionsof said conductive strips are V-shaped and are formed of a resilientmaterial so as to yieldably accommodate the terminals of said fuse. 10.In a cell pack for electrochemical storage cells, havinga casing with apair of terminals at the exterior thereof for connection to an externalcircuit, and a plurality of electrically interconnected cells, theimprovement comprising: at least two of said cells defining a spacebetween said at least two of said cells; a circuit breaking elementcarried by the casing; interconnecting means inside the casing forconnecting the cells in a series circuit between the exterior terminals,said interconnecting means including a pair of isolated contacts, eachof said pair of isolated contacts having a terminal receiving portion,said terminal receiving portion being positioned in said space betweensaid at least two of said cells; said circuit breaking element beingconnected to said terminal receiving portions of said isolated contactsso as to complete the series circuit and to interrupt said circuit whenthe current therethrough exceeds a predetermined level, said circuitbreaking element being positioned in generally parallel relation to saidcells and at least partially occupying said space between said at leasttwo of said cells; and a removable casing portion supporting saidcircuit breaking element, said circuit breaking element having terminalsin physical registration with, and electrically contacting, saidisolated contacts when said removable casing portion is in place. 11.The cell pack of claim 10 wherein:said isolated contacts are formed soas to resiliently receive the terminals of said circuit breakingelements.